How to separate texture-alpha without premultiplying the color?
The workaround WorldSEnder mentioned above is the only solution:


  Duplicate each Texture Input node and get the RBG value from one, and the Alpha value from the other input.


The stance here seems to be that texture-files are simply not expected to contain unrelated information. The optimization to pack several unrelated channels into one texture-file to improve memory-locality on the GPU is just not that relevant to Blender to warrant a change to the behavior of Texture Input nodes.